This invention relates to dual tone multi-frequency (DTMF) receivers.
DTMF receivers are used in the telephone industry, for example in central offices, to detect the presence of pairs of tones at predetermined frequencies. In North America, each pair, to be valid, must comprise one tone from a so-called "low" group of frequencies--697 Hz, 770 Hz, 852 Hz and 941 Hz. The other tone must be from a so-called "high" group--1209 Hz, 1336 Hz, 1477 Hz and, optionally, 1633 Hz.
Hitherto, DTMF receivers have detected such tone pairs by means of a bank of bandpass filters to estimate the energies of narrow bands of the spectrum to obtain a representation of the signal system. A DTMF signal (tone pair) is registered as present if, and only if, the outputs of the bandpass filter bank indicate a sufficicient level of energy in only two valid spectral bands, subject to other criteria generally accepted in the industry. Once the presence of the tone pair has been determined in this way, it merely remains to identify the particular frequencies concerned.
This technique could be used with either analogue or digital bandpass filters. To implement a suitably sharp bandpass filter digitally, however, would require a high filter order and consequentially large computational power. Nevertheless, it is preferable to use digital techniques, especially if it is desired to use a DTMF receiver for secondary signalling, for example in a voice messaging system.
As an alternative to bandpass filters, the shape of the signal spectrum can be determined directly by computing, for example, the correlation of the signal with tones at frequencies of interest. To be reliable, however, the correlation must be computed on long blocks of signal samples, usually referred to as data frames. Furthermore, the correlation must be computed at a large number of frequencies to give a meaningful representation of the signal spectrum. This results in high computational complexity. Such complexity could be tolerated in the past, when DTMF receivers were used only for primary signalling--typically located at a central office and accessed only when a user dialled a destination number.
As mentioned previously, DTMF tones may now be applied to secondary signalling, enabling the user to access services such as voice messaging. It is desirable, therefore, to use DTMF receivers at a local level, for example in, or accesible from, a PBX (private branch exchange). Generally, then, DTMF receivers will be more widespread than hitherto and there is a concomitant requirement for them to be simpler and cheaper.
It has been proposed to reduce the number of computations by applying a modelling technique to the data frame. More specifically, it has been proposed to use Linear Predictive Coding (LPC) to convert the data frame to a set of parameters, i.e. poles, which comprise a model for the signal over the time period of the data frame. Thus, a data frame of 128 samples might be modelled by only 14 LPC poles, or fewer.
The use of LPC in this way is disclosed by B.I. Pawate, W. Steenaart and B. Sankur in a paper entitled "The DTMF Receiver Based on Linear Prediction", Proc. of the Twelfth Biennial Symposium on Communications, Queen's University, Kingston, June 4-6, 1984.
In theory, since the linear predictive coding provides a model of the signal spectrum, the DTMF tones could be detected by applying filtering to the LPC model, computing frequency response magnitudes at the eight different frequencies specified for DTMF signals, and comparing them with thresholds.
In practice, such a simple algorithm does not necessarily indicate the presence of a DTMF tone pair with the reliability and accuracy required within the telephone system.